Salmonella enterica serovar Typhi causes disease in millions of individuals who ingest contaminated food or water. Ingested S. enterica serovar Typhi organisms infect macrophages in the Peyer[unreadable]s patches, which then carry the infection to other organs including the liver and the spleen. This results in either systemic disease, clearance of the organism, or an asymptomatic carrier state in which individuals shed the organism for life. Which of these outcomes will occur depends in large part on the status of the infected individual[unreadable]s CD4+ T cells. Current vaccines consisting of killed or attenuated organisms provide some degree of protective immunity, but this immunity is not completely effective. Thus, a need exists for a better vaccine. We believe that the best way to accomplish this goal is by understanding natural immunity in asymptomatic carriers. Our goal is to do this by studying immunity to S. enterica serovar Typhimurium (ST) in mouse strains that naturally develop a persistent asymptomatic infection of phagocytes that is similar to the carrier state in humans. During the last funding period we developed a sensitive new technique to detect polyclonal peptide-major histocompatibility complex class II molecule (pMHCII)-specific CD4+ T cells and used it to track CD4+ T cells expressing STpMHCII-specific T cell receptors (TCR) in these mice. We found that these T cells underwent massive expansion in the spleen and lymph nodes following intragastric infection, uniformly differentiated into [unreadable]multifunctional[unreadable] IFN-&#947;/TNF-producing Th1 cells, persisted at a high and stable level for hundreds of days, and controlled the infection without disease. We will determine whether these T cells are necessary and sufficient for protection and identify the mechanisms by which they are protective. We will also study how multifunctional Th1 cells are generated and avoid exhaustion during persistent ST infection with the idea that recurrent TCR stimulation by STpMHCII+ phagocytes in the lymphoid organs harboring infected phagocytes interspersed by periods of rest in uninfected tissues is involved. We will attempt to produce these conditions with a vaccine composed of slowly biodegradable poly(&#949;-caprolactone) rods impregnated with antigens, peptides, adjuvants, and inhibitors of negative immune regulation. This research is significant because it could set the stage for a long sought after vaccine that operates via T cell mediated immunity. It is innovative because it is driven by the idea that anatomically confined chronic infection is the best way to generate multifunctional Th1 cells capable of controlling intracellular bacterial infections and because it will be pursued with novel T cell tracking technology.